The invention relates to a semiconductor integrated circuit device, and more particularly to a protection circuit device for an internal semiconductor integrated circuit device.
In general, many semiconductor integrated circuit devices are sometimes forced to suffer the danger of breakdown due to excessively high voltage. The semiconductor integrated circuit device tends to be charged by a static electricity thereby generating any excessively high voltage which is applied to the semiconductor integrated circuit devices. When the semiconductor integrated circuit device is in contact with an article or a human body or the like, an excessively high voltage which has been generated by the static electricity is applied to one or more input pins involved in the semiconductor integrated circuit device. The application of such excessively high voltage caused by the static electricity to the pins causes a relatively large current to be applied to the semiconductor integrated circuit device thereby resulting in a breakdown thereof.
Then, it is necessary to combat such problems in the breakdown of the semiconductor integrated circuit device caused by a discharge of the static electricity. Well known in the art to solve such problems is use of protection circuits which serve to protect the internal semiconductor integrated circuit device from exhibiting a breakdown. Thus, the protection circuit device keeps the internal semiconductor integrated circuit device from being supplied with any excessively high voltage caused by external factors such as the static electricity. To accomplish such protection feature, the protection circuit device involves a pin which is grounded to a ground potential. Such protection circuit device including the grounded pin is provided between the internal semiconductor integrated circuit device and each input pin thereof. When an excessively high voltage caused by the static electricity is applied to any of the input pins, the protection circuit device provided between the input pin and the internal semiconductor integrated circuit device makes a relatively large current caused by such excessively high voltage to be discharged through a grounded discharge pin so as to prevent the current to flow in the internal semiconductor integrated circuit device.
The conventional protection circuit device including a grounded pin for an internal semiconductor integrated circuit device will be described with reference to FIG. 1A. The conventional protection circuit device is arranged between an input pin 11 and an internal semiconductor integrated circuit device. FIG. 1A illustrates an equivalent circuit of the conventional protection circuit device. The equivalent circuit of the conventional protection circuit device has two resistances connected in series between the input pin 11 and the internal semiconductor integrated circuit device. The equivalent circuit of the conventional protection circuit device also has two transistors Q1 and Q2, both of which serve as switching devices. The transistor Q1 is connected at its gate electrode between the resistance and the input pin 11. The transistor Q1 at one of its source and drain electrodes is also connected to one of the resistances which exists between another resistance and the input pin 11. The transistor Q1 at another of the source and drain electrodes is also connected to a grounded pin 13 which supplies the ground potential. The transistor Q2 is connected at its gate electrode between the grounded pin 13 and the another of the source and drain electrodes of the transistor Q1. The transistor Q2, also at one electrode of its source and drain electrodes, is connected to the another of the resistances. The transistor Q2 also at another of the source and drain electrodes is connected between the grounded pin 13 and the another of the source and drain electrodes of the transistor Q1.
The operation of the equivalent circuit of the conventional protection circuit device will be described with reference to the equivalent circuit. When a positive excessively high voltage caused by external factors such as the static electricity is applied to the input pin 11, the transistor Q1 is biased in the forward direction by such positive high voltage and further its gate is also supplied with such positive high voltage. As a result, the transistor Q1 turns ON. Thus, a relatively large current caused by such positive high voltage flows from the input pin 11 through the transistor Q1 to the grounded pin 13 as the grounded pin 13 supplies the ground potential. Namely, the transistor Q1 serving as a switching device forces the relatively large current be discharged through the grounded pin 13. This prevents such current to flow in the internal semiconductor integrated circuit device. The protection circuit device is, therefore, able to make the internal semiconductor integrated circuit be free from a breakdown caused by the application of the large current.
In contrast, when a negative excess high voltage is applied to the input pin 11, the transistor Q1 is biased but in the reverse direction by such negative high voltage and further its gate is also supplied with such negative high voltage. As a result, the transistor Q1 remains OFF. However, the transistor Q2 is biased in the forward direction by such negative high voltage and further its gate is also supplied with such negative high voltage. As a result, the transistor Q2 turns ON. Thus, a relatively large current of negative carriers or electrons caused by such negative high voltage flows from the input pin 11 to the grounded pin 13 but through the transistor Q2. Namely, the transistor Q2 serving as a switching device forces the relatively large current of negative carriers or electrons to be discharged through the grounded pin 13 which supplies the ground potential. This prevents such current to flow in the internal semiconductor integrated circuit device. The protection circuit device is, therefore, able to make the internal semiconductor integrated circuit be free from a breakdown caused by the application of the large current.
In recent years, the semiconductor integrated circuit device is required to have a large capacity and multiple functions. Developments in the large capacity and the multiple functions make the semiconductor integrated circuit device have a plurality of protection circuits independent from each other, each of which has a grounded discharge pin. The grounded discharge pins involved in the protection circuit devices are made to be independent from each other and thus are not connected to each other. Namely, a plurality of protection circuit devices, each of which has a grounded pin, are electrically separated from one another.
Such a semiconductor integrated circuit device has the following disadvantages. When each of the independent protection circuit devices involved in the semiconductor integrated circuit device is normally operative, of course there exists no problem. But, if a disconnection of one of the plural grounded pins from the ground occurs, the grounded discharge pin 13 disconnected from the ground assumes a floating state as illustrated in FIG. 1B. The protection circuit device having the floating discharge pin 13 is unable to exhibit a normal performance as a protection circuit. Since such floating discharge pin 13 is also unable to supply the ground potential. Then, in the protection circuit device including the floating discharge pin 13, if a positive or negative excessively high voltage caused by the static electricity is applied to the input pin 11, a relatively large current generated by the positive or negative excess high voltage is unable to be discharged through the floating discharge pin 13. Thus, the floating discharge pin 13 forces the large current to flow through the two resistances to the internal semiconductor integrated circuit device. The application of the large current to the internal semiconductor integrated circuit device causes the failure of any elements such as a transistor involved in the internal semiconductor integrated circuit device. Such internal semiconductor integrated circuit device including any failed element no longer exhibits a normal performance. The important problem with the floating pin 13 is as follows. If only one of the plural grounded pins provided to the single internal semiconductor integrated circuit device takes a floating state, the single internal semiconductor integrated circuit device suffers the application of the large current thereby resulting in losing a normal performance. Even if all the other pins are grounded and are able to supply the ground potential, the floating of only one of the plural grounded pins forces the internal semiconductor integrated circuit device to be supplied with a large current thereby losing its normal performance. This makes the internal semiconductor integrated circuit device having a plurality of discharge pins suffer a high possibility a danger of losing the normal performance due to the failure of the elements.
To combat the above disadvantage, it is desirable to provide a novel structure of the protection circuit device which is able to exhibit a protective performance for the internal semiconductor integrated circuit device, even if one of the plural discharge pins takes a floating state.